Current tire sizes on my '08 coupe are

275-35-ZR18  front
345-30-ZR19  rear

What are the maximum tires sizes possible on front and rear tires that won't rub against frame?

Will 345 25 ZR20 fit the rear wheels?

You're putting 20's on your SSG?
Which tire are you buying ? Pirelli PZero Rosso?

They'll fit provided your new wheels don't have a screwy offset.

Hey Doc,

Go to the Tire Rack website and check the specs on the tires; height, section width, tread width, etc.

As Lance said, be very careful to get wheels with the stock offset.  Just a little bit wider offset and you will have fitment issues.

Also, non-stock size tires will likely affect your speedometer unless they are exactly the same height, i.e. 27.2 inches tall in 2008.

At the track, I run Hoosier R6s in 315 x 30 x 18's H up front.  This is about the max width on the front of a SRT. 
I run  Hoosier R6s in 345 x 35 x 18 at the back on the track (26.8 tall).  At the drags, I run Nitto NT05R drag radials in 345 x 30 x 19 which are 27.4 tall.

IMHO, it's very hard to beat the PS2 OE tires in the C1 sidewall designation for max all around performance, i.e dry traction, wet traction, ride quality, durability, etc.

Dan

 Hey Doc,

The 345/25/20 should fit the rear, I ran Pirellis with no problems on my HRE wheels, Im
getting ready to put some Nitto Invos in the same size on the rear.

So what kind of wheels are you getting?

grazie mille guys.  My garage is filling up with all kinds of tires